speedo accuracy

[abatzis2@hotmail.com (Michael Abatzis)]

this is what i did, except i used a stopwatch, timed it at 85 mph, and did 
it over 10 miles.  i think my calculated speed was probably pretty damn 
close to actual speed.

-Michael Abatzis
Hotlanta! GA
1988 Scirocco 2L 16v RIP-->parting, prelim web site, but not taking orders 
yet: www.learnlink.emory.edu/~mabatzi/mikes_page1.html
1987 Scirocco 2L 16v...




>From: "Brandon Smith" <SCIROCCO_SPEED@msn.com>
>To: "SCIROCCO-LIST" <scirocco-l@scirocco.org>
>Subject: Re: speedo accuracy
>Date: Sat, 14 Dec 2002 17:07:31 -0700
>
>Okay guys, lets keep it real simple and do some math with the Distance/Time
>equation.  Measure out exactly 1 mile.  At the very beginning of that one
>mile, lay down a simple pressure sensitive cable, or light emitting object
>with a reflector on the other side of the road, and either one rigged to a
>timer.  Down the road at the end of the one mile, have the same thing that
>stops the timer.  Just get a good start and have a constant velocity of say
>75mph for example so that when you pass through the first photo-gate it
>would (for all intents and purposes) be instantaneous, or even go by RPM.
>
>Okay so maybe its not that simple, but aren't there a few physics teachers
>on this list?  They should have adequate equipment to accomplish this.
>
>This gets me thinking about the Tire Slip idea.  It makes sense and I
>understand it, but wouldn't it take place more so when accelerating or
>decelerating, seems that when traveling at a constant velocity it wouldn't
>happen that much to affect a speedo.
>
>just my tired thoughts,
>
>-Brando
>
>
>----- Original Message -----
>From: "Rabbit16v" <Rabbit16v@attbi.com>
>To: "Scirocco List" <scirocco-l@scirocco.org>
>Sent: Saturday, December 14, 2002 1:00 PM
>Subject: Re: speedo accuracy
>
>
> > Your only fault in all of this is that you haven't had it radar checked.
> > I'll bet if you did have it checked with radar that it would be off just
> > like everyone else's.  I guess the problem that we need to figure out is
> > where our calculations are going wrong.  At this point I think we need 
>to
> > question the charts we have access to on the internet?  Anyone have a
>chart
> > on paper in a manual or something that they can compare to the charts on
> > Scirocco.org?
> >
> > Dave
> >
> > >
> > > My $.02 CDN on the speedo issue (along the same lines as Ron's)  Both
>the
> > > Scirocco and the winter-beater Jetta have 7A transmissions in them.  
>The
> > > Scirocco has the stock 220KM/h speedo, and the tach is a W-field
> > > alternator driven unit from an 89 Jetta TD (ranging from 0-5000rpm)  
>The
> > > Jetta has a 16V speedo (260km/h) and tach (0-8000rpm)  The speedo 
>drive
> > > gear matches the R&P of the transaxle (3.94 in the 7A)
> > >
> > > These two cars read as dead on as I can reckon by eye for RPM vs.
> > > indicated speed on the speedometer - ie. 100KM on the clock is 2575 
>rpm
>on
> > > both tachs (admittedly harder to pinpoint on the 8K tach due to
>scale...)
> > > which impresses me, especially as the Scirocco's tach runs from the
>pulses
> > > on the alternator, not the ignition, which means it's more prone to
>error
> > > (belt slip, different pulley diameters etc)
> > >
> > > I know that the Scirocco's actually speed will be off very slightly
> > > (compared to the indicated speed) because it has 185/60R14, which
>results
> > > in a slight increase in travelled speed (the rpm difference at 100km/h
>is
> > > less than 50 though)
> > >
> > > Personally I'm impressed by the consistency of VW's speedos, even if
>they
> > > are optomistic (which I have not had occasion to determine.)  The fact
> > > that these two cars with different tachs and speedos come up with the
>same
> > > readings as can be calculated from gear ratio and tire diameter is 
>cool.
> > >
> > > Drew
> > >
> > >
> > >
> > >
> > >
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> >
> >
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>
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